gh-156234: Fix and rewrite the curses documentation on reading (GH-156235)

Fix wrong types: instr() and getstr() return a bytes object, not a str, and
their n limits the number of bytes; getkey() returns a str; unctrl() returns
a bytes object.  Make clear whether an integer standing for a character is an
encoded byte or a character code.

Rewrite the documentation of getch(), get_wch(), getkey(), getstr(),
get_wstr(), instr(), in_wstr() and in_wchstr(), following X/Open Curses.